1. Field of the Invention
The present invention relates to an apparatus and method for re-tensioning a loose roof plate that has become loose in an underground mine, such as a coal mine.
2. Description of Related Art
Surface mining and deep underground mining are the two basic methods of mining coal. Coal seams relatively close to the surface are usually surface mined, whereas coal seams occurring at greater depths are usually mined underground. Underground mining accounts for most of world's coal production.
In underground mining, roof bolting has been an important technological development for supporting the roof of an underground mine. In roof bolting, bolts are installed within a rock mass to reinforce and stabilize the rock formation above the mine tunnel. Conventionally, it is common practice to drill a hole through a mine tunnel ceiling into the rock formation above. A roof plate is then mounted on to the head of a roof bolt that is installed into the hole. The roof plate is then brought up against the mine ceiling as the roof bolt is tightened into place, allowing the roof plate to applying a pressure against the roof of the tunnel.
Over time, the roof layer just above the roof plate may crumble and flake over time, causing the roof plate to become loose. Support of the roof is compromised because the loose roof plate is no longer applying pressure against the roof of the tunnel. Related art tensioning devices have attempted to solve this problem by reestablishing contact between the roof and the loose roof plate.
U.S. Pat. No. 5,733,069 to Schofield describes an apparatus and method for retensioning a loose roof plate that is held in place by a roof bolt in an underground mine. The device includes an externally threaded split bushing, a spacer unit and an internally threaded nut. To retension the loose roof plate, the externally threaded split bushing is placed around the roof bolt to be in contact with the roof bolt head and between the roof bolt head and the loose roof plate. The spacer unit is then slid over the roof bolt head and over the externally threaded split bushing to be placed into contact with the loose roof plate. The internally threaded nut is then slid over the roof bolt head and threaded onto the externally threaded split bushing until the roof plate is snug against the roof. However, one problem of the Schofield device is the difficulty of threading the internally threaded nut onto the externally threaded split bushing. Another problem is the difficulty of manufacturing.
U.S. Pat. No. 6,637,980 to Robertson describes an apparatus and method for re-tensioning a loose roof plate that is held in place by a roof bolt in an underground mine. The apparatus includes a slotted screw member, a slotted nut and optionally one or more slotted spacers. To re-tension the loose roof plate, the slotted screw member and slotted nut are coupled together, placed around the roof bolt between a head of the roof bolt and the loose roof plate with the head portion of the screw member oriented toward the roof bolt head. Then, the slotted screw member and slotted nut are turned away from each other such as to bias the roof plate against the mine roof. If a gap between the roof bolt head and the roof plate is great, then one or more slotted spacers are placed between the roof bolt head and the head portion of the screw member. However, one problem of the Robertson design is that it cannot be used when the gap between the roof bolt head and the roof plate is less than the length of the slotted screw member. Another problem is the difficulty of manufacturing.